STN-SR in SRVCC

STN-SR (Session Transfer Number for SRVCC), is a temporary directory number that uniquely identifies the user's active voice call during the SRVCC handover.

The HSS (Home Subscriber Server), or the MME (Mobility Management Entity) in the 4G/LTE network can allocate this number. And then they send this number to the UE and the target 2G/3G network during the SRVCC handover preparation process.

So that the STN-SR number ensures that the active voice call can seamlessly transfer from the 4G/LTE network to the 2G/3G circuit-switched network. Which provides call continuity for the user.